Public Switched Telephone Network is the standard for how telephone numbers work. There is the internation country code such as +1 for the US, or +44 for the UK. Then each country designates the domestic number as they see fit. The number must range from 6-12 digits long. Therefore the maximum a telephone number can be is 15 digits (3 digits for country code + 12 digit domestic number). Every country has an international access code which is 011 in the US/Canada or 00 through most of the world. This number will dial first before dialing such an international number. Therefore, the subscriber number can not begin with this number. In example, 00 is the international country code for the UK. Domestic calls starts with 0. That second digit can not be a 0. When dialing a country whose domestic number starts with a 0. that 0 is omitted when dialing in from outside of the country.